PROJECT SUMMARY (ABSTRACT) The obesity epidemic has greatly increased the incidence of non-alcoholic fatty liver disease (NAFLD), the condition now identified as the leading cause of chronic liver injury in the United States. NAFLD is defined by excess accumulation of lipids in the liver, also referred to as steatosis. Prolonged fatty liver is highly associated with increased cell injury, inflammation, regrowth, and fibrosis, a stage referred to as non-alcoholic steatohepatitis (NASH). Advanced NASH often leads to cirrhosis, an end-stage liver condition which necessitates liver transplant in patients. Furthermore, repeated insults to liver tissue results in accelerated cell turnover, raising the risk of mutations and progression to hepatocellular carcinoma (HCC), the third leading cause of cancer-related death worldwide. Despite a clear association between NAFLD and progression to these deadly sequelae, the pathogenesis of these conditions remains poorly understood. The impending contributions that rising obesity rates will make to NAFLD prevalence makes elucidating its pathogenesis urgent. Our group previously characterized a novel adipose-secreted protein Neuregulin-4 (Nrg4) which targets liver to improve steatosis. Interestingly, Nrg4 expression is downregulated in NASH, implying a possible role of Nrg4 in regulating NASH pathogenesis. Our preliminary studies showed that Nrg4 deficiency renders mice susceptible to NASH. These mice exhibited increased apoptosis and necroptosis in liver, which are central pathways in the pathogenesis of NASH. Interestingly, these mice displayed lowered levels of c-FLIPL, a key regulator of these pathways. Our findings indicate Nrg4 may prevent NASH through attenuation of cell death pathways. Despite these provocative findings, the potential for Nrg4 signaling to block the progression of NAFLD to NASH and HCC remains unexplored. The goal of this study is to unravel the role of Nrg4 signaling in protecting from NASH and HCC pathogenesis. Here, we propose to elucidate the effect of Nrg4 on NAFLD progression with the following aims: Aim 1 will evaluate the effect of excess Nrg4, in a transgenic context, and also exogenously administered, in alleviating NASH in mice. Aim 2 will utilize recombinant Nrg4 treatment of primary hepatocytes to dissect the signaling pathways regulating Nrg4 attenuation of cell death, and harness viral expression to test if modulating these pathways could rescue Nrg4 deficient mice from NASH. Aim 3 will evaluate the potential for Nrg4 to prevent NASH-associated HCC using both Nrg4 transgenic and knockout models. In summary, we anticipate these aims will potentially uncover a novel endocrine pathway which protects from NASH and HCC, and may provide a therapy for these conditions.